The typical structure of an interline CCD pixel 5 is shown in FIG. 1. It consists of a photodiode 10 adjacent to a CCD 20. The photodiode 10 converts incident light photons into charge. The transfer of charge through the CCD 20 is generally controlled by at least two control gates, 40 and 50. One of the gates 50 has a transfer gate region 30 which controls the transfer of charge from the photodiode 10 to the CCD 20. The pixels 5 are arranged in a two-dimensional array of at least one row and one column.
FIG. 2 shows a cross section A-B of FIG. 1. This cross section shows the additional detail of the pixel 5 including the gate insulator 70 separating the gate 50 from the CCD 20. There is also a light shield 60 to prevent photo charge from being generated in the CCD 20. The photodiode 10 charge is cleared by applying a high voltage pulse to the substrate 80.
The typical sequence of events in a camera to acquire an image using an external shutter are (a) open external shutter to start the exposure time; (b) wait for the desired exposure time; (c) close the external shutter to end the exposure time; (d) transfer charge from the photodiode 10 to the CCD 20; and (e) transfer charge through the CCD 20 by clocking the control gates 40 and 50 to move charge towards a charge measurement structure.
Sometimes very short exposure times are desired, i.e., much shorter than what can be obtained by an external shutter. In this case, the electronic shuttering capabilities of an interline CCD are used. The typical sequence of events to acquire an image using the electronic shutter are (a) open external shutter; (b) pulse the substrate 80 voltage to clear the photodiode 10; (c) wait for the desired exposure time; (d) transfer charge from the photodiode 10 to the CCD 20 to end the exposure time; (e) close the external shutter; (e) transfer charge through the CCD 20 by clocking the control gates 40 and 50 to move charge towards a charge measurement structure. When using the electronic shutter every pixel must transfer its charge from the photodiodes to the CCD simultaneously. The simultaneous charge transfer prevents unwanted artifacts when capturing an image of a fast moving object. The charge capacity of the photodiodes must be designed to be smaller than the charge capacity of the CCD. If the photodiodes contain more charge than the charge capacity of the CCD, the CCD will overflow. This is commonly referred to as blooming image defects.
FIG. 3, which is a cross section from point A to point C in FIG. 1, illustrates how blooming occurs in the CCD. The cross section begins in the photodiode 10, goes through the transfer gate region 30, and then up the CCD 20. The CCD in FIG. 3 also shows barrier implants 90 and 95 which are used to modify the potential energy in the CCD channel 20 to control the direction of charge transfer. At time T1 of FIG. 3 the CCD is empty and the photodiode contains a full charge packet. In this case, to illustrate blooming, the charge packet is larger than the capacity of the CCD. At time T2 the gate 50 over the transfer gate region is set to a high voltage level which lowers the barrier between the photodiode and CCD. All charge flows from the photodiode into the CCD. Then at time T3, the gate 50 is returned to its normal low level. Since the charge packet does not fit entirely under gate 50, charge spills backwards into gate 40.
The invention described here overcomes the problem of the photodiodes containing more charge than the charge capacity of the CCD that leads to blooming image artifacts.